The present invention relates generally to cathode-ray tube (CRT) graticules, and more particularly to a CRT incorporating multiple internal graticules, only one of which is visible in normal ambient light.
As is well known, oscilloscope cathode-ray tubes are normally provided with a reference scale, or graticule, for making quantitative measurements. The graticule may for example, be engraved or printed on a transparent plastic plate mounted in front of the CRT's faceplate. Such "external" graticules are spaced at least the thickness of the faceplate from the tube's phosphorescent image display screen, making measurements susceptible to parallax error. For that reason, it is preferable to form the graticule on the inner surface of the faceplate so that the scale is in the same plane as the phosphorescent screen.
A principal drawback of such "internal" graticules is that they cannot be changed. This is a significant problem in certain special purpose oscilloscopes, such as those used to measure television signals. Television signals are quite complex, and a variety of different measurements must be made to characterize them completely. For example, conventional waveform displays are used to determine signal amplitude, bandwidth, linearity and distortion, signal-to-noise ratio, differential gain and signal component crosstalk. Vector presentations are employed to measure phase-related signal characteristics, such as chrominance phase and amplitude and to determine color errors arising during color encoding, video tape recording or signal transmission. Frequency response errors, phase response errors and the existence of nonlinear errors can be revealed by transient waveform analysis using pulse test signals.
It will be understood that several different scales are needed to make these measurements efficiently and conveniently. In an instrument using external graticules, the scales can be provided on individual transparent plates for selective mounting in front of the CRT screen. However, as already mentioned, external graticules are undesirable because of the parallax problem. A single internal graticule combining the various measurement scales would have a cluttered, difficult-to-use and error-phone pattern. An alternative approach disclosed in Japanese Utility Model Publication No. 7058/73 is the use of a combination of internal and external graticules. The external graticule is mounted on the faceplate at all times, and is edge-lighted selectively (as is the internal graticule) when needed. In this arrangement, however, measurements are parallax error-free only when the internal graticule is in use. An additional drawback is that a relatively cumbersome mechanical arrangement is needed for mounting the external graticule and two edge-lighting lamp assembles.
A general object of the present invention is to provide an improved multiple graticule construction that is free from the drawbacks of the above-described alternatives.
Another, related object is to provide a cathode-ray tube that incorporates multiple internal graticules.
A more specific object is to provide a CRT having at least two internal graticules, only one of which is easily visible in normal ambient light.
A still more specific object is to provide a CRT that includes two internal graticules, one of which is always visible and the other of which is readily visible only when edge lighted.
Additional objects, features and advantages of the present invention will become apparent as the following detailed description is read in conjunction with the accompanying drawing.